In the isolation of vibration between structural components, it has become well known to use a resilient bushing having a pair of concentric rigid, typically metal sleeves. The inner sleeve is secured to one structural component, and the outer sleeve is secured to the other structural component. An annular elastomeric insert is concentrically positioned between the rigid sleeves typically under radial compression. Such resilient bushings are also utilized to increase the dampening of metal structures such as the frames of automobiles, and to interrupt low impedance all-metal paths for the transmission of structure-borne sounds in a metal structure. Illustrative of previously known resilient bushings and machines for making such bushings are those described in U.S. Pat. Nos. 2,749,160, 2,824,362, 2,840,893, 2,844,398, 2,858,155, 2,872,727, 2,877,543, 2,895,215, 3,082,999, 3,147,964, 3,171,699, 3,239,286, 3,380,791, 3,387,839, 3,560,034 and 3,643,320.
Originally these resilient bushings are made by inserting the uncured elastomeric insert between the concentric sleeves and thereafter relieving the internal stresses of the insert during bonding and curing. To increase the load-bearing capacity of the bushings, precured inserts were compressibly inserted between the sleeves. It was also found that various spring rates in different radial directions could be achieved in a single bushing by providing the insert with various recesses.
One of the continuing difficulties with such resilient bushings has been fatigue life. That is, the failure of the bushing after it has flexed a given number of vibrational cycles. Fatigue failure is caused by the relative movement between the rigid members and the elastomeric insert and the resulting wearing away of the elastomeric insert. Thus, extended fatigue life has been possible only where there is no relative movement between the metal sleeves and the elastomeric insert. Compressive insertion of precured elastomeric inserts between the sleeve members has increased fatigue life. However, it still remains common to specify a required fatigue life in terms of a lower number of cycles e.g 100 kilocycles, before failure.
Previously, resilient bushings have been made with phosphate coated metal sleeves. Phosphate coatings have been specified for manufacturing purposes. The phosphate coating aided in assembly by holding a lubricant and preventing rust and corrosion of the sleeves after assembly. The coating weight and the corresponding surface roughness of the coating varied with the composition of the phosphate coating bath (e.g. anion, accelerator, etc.), the temperature of the coating bath, and the length of the coating cycle. However, for practical reasons, the phosphate coating utilized has typically been between 700 and 900 milligrams/ft.sup.2. Heavy phosphate coatings of greater than 2,000 milligrams/ft.sup.2 have been used to aid assembly and provide increased interference fit between the outer member and a control arm, etc., in certain applications. But, the corresponding surface roughness height rating has not exceeded 170 RMS measured by SAE Standard J448a and has not been considered to have lasting effects because of the smoothing of the phosphate coating on assembly and subsequent use.
The present invention extends the fatigue life of resilient bushings by preparation of the surfaces of the rigid members contacting the elastomeric insert. The preparation provides a surface roughness greater than previous bushings by sandblasting to reduce the movement between the rigid members and the elastomeric insert.